Increase in fine root biomass enhances root exudation by long-term soil warming in a temperate forest
Jakob Heinzle, Xiaofei Liu, Ye Tian, Steve Kwatcho Kengdo, Berthold Heinze, Annika Nirschi, Werner Borken, Erich Inselsbacher, Wolfgang Wanek, Andreas Schindlbacher
Abstract
Trees can invest up to one-third of the carbon (C) fixed by photosynthesis into belowground allocation, including fine root exudation into the rhizosphere. It is still unclear how climate and soil warming affect tree root C exudation, in particular quantifying longer-term warming effects remains a challenge. In this study, using a C-free cuvette incubation method, in situ C exudation rates from tree fine roots of a mature spruce dominated temperate forest were measured in regular intervals during the 14th and 15th year of experimental soil warming (+ 4°C). In addition, a short-term temperature sensitivity experiment (up to + 10°C warming within 4 days) was conducted to determine the inherent temperature sensitivity of root exudation. Root exudation rates in the long-term warmed soil (17.9 μg C g –1 root biomass h –1 ) did not differ from those in untreated soil (16.2 μg C g –1 root biomass h –1 ). However, a clear increase (Q 10 ∼5.0) during the short-term temperature sensitivity experiment suggested that fine root exudation can be affected by short-term changes in soil temperature. The absence of response in long-term warmed soils suggests a downregulation of C exudation from the individual fine roots in the warmed soils. The lack of any relationship between exudation rates and the seasonal temperature course, further suggests that plant phenology and plant C allocation dynamics have more influence on seasonal changes in fine root C exudation. Although exudation rates per g dry mass of fine roots were only marginally higher in the warmed soil, total fine root C exudation per m 2 soil surface area increased by ∼30% from 0.33 to 0.43 Mg C ha –1 yr –1 because long-term soil warming has led to an increase in total fine root biomass. Mineralization of additional fine root exudates could have added to the sustained increase in soil CO 2 efflux from the warmed forest soil at the experimental site.